<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>伪元素选择器</title>
  <style>
    /* 
    ::first-line
    ::first-letter  
     */

     
    div {
      width: 100px;
      height: 100px;
      background-color: blue;
    }
    div::after {
      content: "你好";
    }
    div::before {
      content: "HELLO";
    }
  </style>
</head>
<body>
  <div>
    ok
  </div>
</body>
</html>